1. Field of the Invention
This invention relates to a rod lens array and also to an optical printer head and an optical printer using such rod lens array. A rod lens array according to the invention is adapted to cause a plurality of light emitting elements to emit light and form highly accurate and sharp images of the elements on the surface of a photosensitive member that operates as a recording medium. A rod lens array according to the invention can suitably be used in an optical printer head or an optical printer comprising an LCD (liquid crystal display elements) or LEDs (light emitting diodes) to name a few.
2. Related Background Art
Rod lens arrays are known as conventional oblong imaging element arrays that are used in liquid crystal printers and LED printers for forming a correct life-size image of an object. A rod lens array is prepared by arranging a plurality of rod lenses cyclically in a predetermined direction, which rod lenses radially exhibit a refractive index distribution.
FIG. 1 of the accompanying drawings shows a schematic cross sectional view of a known rod lens array taken along a direction perpendicular to the optical axes of the rod lenses. In FIG. 1, reference symbol 30 denotes a rod lens array and reference symbol 31 denotes a rod lens. A rod lens 31 is normally formed by subjecting a rod-shaped piece of glass to an ion-exchange process so as to make it exhibit a refractive index distribution. A plurality of rod lenses 31 are densely arranged in the horizontal direction (hereinafter referred to as main-scanning direction) in two rows that are disposed one on the other in the vertical direction (hereinafter referred to as sub-scanning direction). Each of the rod lenses 31 is adapted to form a correct life-size image of an object at a position separated from it by a predetermined distance.
The plurality of rod lenses 31 are densely arranged and aligned relative to each other by means of a pair of side panels 32. Subsequently, the gaps separating the rod lenses 31 are filled with opaque resin 33 to securely hold the rod lenses in position.
Normally, the side panels 32 are made of fiber-reinforced plastic or FRP. Therefore, the surfaces of the lateral walls 32 generally show undulations of a dimension of 6 to 8 xcexcm reflecting the profile of the interwoven fibers contained therein.
On the other hand, known rod lens arrays are often accompanied by a problem of disorder of lens arrangement as seen from FIG. 1 that arises in the manufacturing process because so many rod lenses are put there.
FIG. 2 is a schematic cross sectional view of a principal portion of an optical printer realized by using such a rod lens array, illustrating the image forming effect of the rod lens array in an ideal state. In FIG. 2, the components the same as or similar to those in FIG. 1 are denoted respectively by the same reference symbols and will not be described any further.
Referring to FIG. 2, light emitting means (an array of light emitting elements) 45 comprises a plurality of light emitting elements that are adapted to emit respective light beams according to the image signal applied thereto. Then, the light beams are focused to respective imaging spots 44 on the surface of a photosensitive drum 46 operating as a recording medium to form a correct life-size image of an object by means of a rod lens array (imaging device) 30 comprising a plurality of rod lenses 31 arranged in an array. With this arrangement, a latent image of the object can be formed on the surface of the photosensitive drum of the optical printer. Note that the optical printer head of the optical printer includes the light emitting element array 45 and the rod lens array 30. It has been believed that such a rod lens array does not produce any disorder of imaging spots on the photosensitive drum 46 if the arrangement of the rod lenses involves disorder to a slight extent because the rod lenses form a correct life-size image.
Meanwhile, in response to the increased demand for high definition printers in recent years, optical printer heads and optical printers providing a high resolution of 600 dpi or 1,200 dpi are currently being marketed. At the same time, while high definition optical printers adapted to deal with binary data have so far been in the main stream, those adapted to produce pictorial color images are appearing in the market. In other words, halftone images are becoming more and more important to optical printers. In line with this trend, means for electrically correcting the problem of unevenness existing in the optical printer including that of an uneven distribution of the quantity of light of light-emitting dots have been devised to improve the image quality of optical printers particularly in terms of halftone images.
However, while efforts have been paid to accurately correct the problem of uneven distribution of the quantity of light, halftone images produced by optical printers have often encountered a problem of creasy marks to degrade the image quality, although the reason for this problem is still unknown. As a result of research efforts, the inventor of the present invention has found that the problem of creasy marks is different from that of the uneven distribution of the quantity of light and attributable to an inaccurate arrangement of rod lenses of the rod lens array of the optical printer.
However, if a perversive factor or a distortion that can partially damage the magnification is involved in any of the rod lenses of the array as shown in FIG. 3, the light beam emitted from the rod lens involving such a problem is not focused at the right spot to give rise to a phenomenon referred to as a broken dot or a plump dot, which can give rise to creasy marks on the produced image at the time of development. FIG. 3 is a schematic cross sectional view of a principal portion of an optical printer realized by using a rod lens array, illustrating the image forming effect of the rod lens array when some of the rod lenses 51 involve a perversive factor. In FIG. 3, reference symbol 50 denotes a rod lens array and reference symbol 52 denotes a side panel, while reference symbols 55 and 54 denote a light emitting element array and a spot formed by a focused light beam, respectively.
Since known rod lens arrays are not required to have a high resolving power, the problem of a broken dot is not serious if it arises. However, in the case of an optical printer head or an optical printer required to have a resolution of 600 dpi or more, any cyclical changes in the formation of dots can result in an uneven density distribution of the obtained image that normally involves half tones to give rise to a serious problem of creasy marks.
The inventor of the present invention also found that, when the side panels are made of FRP that has a cyclical structure, the uneven density distribution of the obtained image is significantly influenced by cycle of the beat produced by the cycle of arrangement of the rod lens array and that of the cyclical structure of the side panels. This invention is based on these findings.
Therefore, it is an object of the present invention to solve the above-identified technological problems of the prior art and provide a rod lens array that is practically free from the problem of degraded image quality due to the uneven density distribution of the obtained image if it is used for an optical printer.
Another object of the present invention is to provide en optical printer head and an optical printer that can produce high quality images by using a rod lens array according to the invention.
According to the invention, the above objects are achieved by providing a rod lens array comprising:
a plurality of rod lenses arranged cyclically in a predetermined direction;
an opaque substance filled in the gaps separating the plurality of rod lenses; and
a pair of side panels sandwiching the plurality of rod lenses and the opaque substance, the side panels having a cyclical structure at least at the sides thereof facing the rod lenses;
the pitch of the beat of the cycle of arrangement of the plurality of rod lenses and the cyclical structure of the side panels being not greater than 0.6 mm or substantially infinite.
According to another aspect of the invention, there is provided an optical printer head comprising:
a light emitting element array formed by arranging a plurality of light emitting elements;
a plurality of rod lenses arranged cyclically in a predetermined direction and adapted to focus the light beams emitted from the light emitting elements of the light emitting element array respectively at positions separated therefrom by a predetermined distance;
an opaque substance filled in the gaps separating the plurality of rod lenses; and
a pair of side panels sandwiching the plurality of rod lenses and the opaque substance, the side panels having a cyclical structure at least at the sides thereof facing the rod lenses;
the pitch of the beat of the cycle of arrangement of the plurality of rod lenses and the cyclical structure of the side panels being not greater than 0.6 mm or substantially infinite.
According to still another aspect of the invention, there is provided an optical printer comprising:
a light emitting element array formed by arranging a plurality of light emitting elements;
a photosensitive member separated from the light emitting element array by a predetermined distance;
a plurality of rod lenses arranged cyclically in a predetermined direction and adapted to focus the light beams emitted from the light emitting elements of the light emitting element array on the photosensitive member;
an opaque substance filled in the gaps separating the plurality of rod lenses;
a pair of side panels sandwiching the plurality of rod lenses and the opaque substance, the side panels having a cyclical structure at least at the sides thereof facing the rod lenses;
a developing unit for developing an electrostatic latent image formed on the photosensitive member by irradiating the photosensitive member with the light beams emitted from the light emitting elements to a toner image;
a transfer unit for transferring the developed toner image onto an image receiving member; and
a fixing unit for fixing the transferred toner image on the image receiving member;
the pitch of the beat of the cycle of arrangement of the plurality of rod lenses and the cyclical structure of the side panels being not greater than 0.6 mm or substantially infinite.